Novon 17: 449. 2007.
Annuals, 10–60 cm. Stems densely branched basally; purple-tinged; glandular-pubescent. Leaves: (stipules scalelike, minute, obscured by pubescence); petiole 1.5–4.5 cm; leaflets 3, blade flat, ovate to obovate, 1–3.5 × 0.5–1.3 cm, margins serrulate-denticulate or entire, apex obtuse, surfaces glandular-pubescent. Racemes 1–3.5 cm (5–40 cm in fruit); bracts unifoliate, obovate to spatulate, 3–25 mm. Pedicels 7–17 mm. Flowers: sepals deciduous, distinct, yellow, awl-shaped, (3–)4–5(–6) × 3–7 mm, margins entire, densely glandular-hairy; petals golden yellow, oblong, 6–12 × 0.3–0.4(–0.6) mm; stamens yellow, 10–17 mm; anthers 1.8–2 mm; gynophore 10–18 mm in fruit; ovary (compressed), 3.5–5 mm; style 1–3 mm. Capsules (pendent) not inflated, 12–25 × 8–12 mm, striate, (glandular-pubescent). Seeds 10–20, brownish black, spheroidal, 3–3.2 mm, (glossy) smooth. 2n = 40.
Phenology: Flowering summer.
Habitat: Alkaline clay soils, volcanic tuff, dry foothills, among junipers, in sagebrush scrub, fields, railroads
Elevation: 800-1200 m
Calif., Idaho, Nev., Oreg.
Peritoma platycarpa resembles some species of Cleomella in the absence of a nectary-disc, shape of the replum, and indument.